Normally use of a sandblaster requires that the flow of abrasive material, such as sand, and compressed air be shut off during operation at times, for example to allow the nozzle operator to change position. Therefore, such machines are provided with air-sand mixture control or shut-off valve means which may be summarized as being relatively complex.
Another equally-important disadvantageous characteristic of sandblasters is that certain internal parts become damaged relatively quickly under the abrasive action of high velocity air-sand mixtures. For example, U.S. Pat. No. 3,557,498, issued to Lindsay, has a sand inlet orifice 51, washer 58 and cushion 57 which can be expected to sustain high wear in a relatively short time, thereby needing frequent replacement and resulting in a loss of work efficiency.